There are a number of different known methods by which relatively large surface areas of an object can be simultaneously measured along x, y and z coordinates by projecting a regular, grating-shaped pattern onto the surface of the object. These methods, which are referred to as "moire topography", include different variants whereby the so-called moire effect (created by light which passes twice through a grating having the same period) is utilized to obtain height information pertaining to the object being measured.
One of these variants is the "shadow pattern" moire method described by Takasaki in Applied Optics 6 (1970), page 1467, in which the object surface to be tested is illuminated by point-source light that is passed through a grating positioned relatively close to the surface. The surface of the object is then viewed through the same grating, but from a point distant from the source of the measuring light, so that the illumination rays and the reflected imaging rays subtend an angle. Inasmuch as the grating pattern projected on the object surface is deformed as a function of the topography of said surface, contour lines are formed as a result of the moire effect as the imaging rays pass through the same grating; and these contour lines provide surface height information. With this method, the contour lines remain visible even if the basic frequency of the grating used for illumination is not resolved at the time of imaging, or even if the grating is "averaged away" by being shifted by one or more full grating periods during the recording of an image.
Another variant of the moire topography method is the so-called "projection moire" method described in U.S. Pat. No. 4,564,295. According to this method, an image of a grating is projected on the object surface, and an image of the object surface is then projected through a lens and a second grating positioned in front of the recording camera. This prior art method permits synchronous shifting of the two gratings--i.e., the projection grating and the grating used for imaging--during the imaging operation, thereby averaging out grating irregularities without changing the contour lines resulting from the moire effect or their location in space. However, this method requires that the gratings have the same grating constant and that the focal lengths of the projecting unit and the imaging lens be the same. This prior art patent further discloses that two projectors can be positioned symmetrically at the same distance and the same angle of projection on either side of the path of measuring rays, i.e., the camera axis. This double projection arrangement generates overlapping and adjoining grating patterns, thereby eliminating the problem of shadows when measuring severely curved object surfaces.
A third variant of the moire topography method dispenses with the second grating at the front of the recording camera and, instead, uses the line raster of the recording video camera or the pixel period of a CCD camera for the function of the second grating. This so-called "scanning moire" method is described in Applied Optics, Volume 16, No. 8 (1977), page 2152.
In addition to these just-described moire topography methods, it is also known that an object can be measured by calculating height information directly from the deformation of a bar grating pattern on the object surface without using a second grating in front of the camera. These so-called "bar projection methods" are described in European Patent No. EP-A2-0 262 089, and in U.S. Pat. Nos. 4,641,972; 4,488,172; and 4,499,492.
These prior art moire topography methods and bar projection methods produce quantitative coordinate measurement information by evaluating the cyclical brightness variations of the resulting bar patterns or contour lines. Generally, this is called phase measurement; and this is usually carried out in such a manner that, during the process of making each measurement, the position of the projection grating is shifted in several steps by fixed increments, often corresponding to a phase shift of 90.degree. or 120.degree..
However, with these known methods, it is relatively difficult to obtain adequate measurements of larger industrial objects, particularly if the surfaces of such objects have irregularities such as edges and tiers. The difficulty arises for the following reason:
Due to the symmetrical perspective arrangement of the projection and viewing rays, the distance of successive contour lines is not constant, but increases with the increasing depth of the object; and without the knowledge of the actual distance of the object surface from the camera at at least one point, it is not possible to obtain data pertaining to the form of a profile of a continuous surface. That is, such conventional methods of phase measurement calculate the object distance only in terms of one complete change of phase and, thus, provide only relative values within one ordinal number of a moire pattern. Therefore, it is not possible with these methods of phase measurement to accurately analyze the sudden large changes in the cyclical bar patterns that occur over the edges and tiers of irregular industrial objects.
In order to increase the range within which bar phase differences can be converted to absolute height measurements without ambiguity, it has been suggested that bar patterns with widely different bar periods be simultaneously projected onto the object surface. Such a system is disclosed in U.S. Pat. No. 4,488,172 in which two projectors with moving gratings create two bar patterns which move over the surface being measured, one of these patterns having a period eight times longer than the other pattern. In order to evaluate the phase relationships between these patterns, the grating used for projection of the longer pattern must be moved at a speed which is eight times faster than the movement of the grating used for the shorter pattern; and this movement results in the loss of a fixed spatial relationship between the two gratings. Unfortunately, such a fixed relationship of the two gratings relative to each other is important when highly accurate measurements are desired.
The apparatus disclosed in European Patent No. EP-A2-0 262 089 generates two moire bar patterns having widely different periodicities. This is accomplished by superimposing two gratings having the same period and then rotating the gratings relative to each other to a varying degree between each projected pattern. The disadvantage to this system, among others, is that it is relatively slow. That is, in order to generate a bar pattern exhibiting a different period, the gratings must be adjusted mechanically, and alternation of the projected bar patterns cannot be accomplished at desired video-cycle speeds.
The invention herein overcomes the just-described prior art problems by providing a method and apparatus of the above-identified type which assure clear, absolute height measurements within a wide measuring range, the measurements being characterized by high accuracy and being obtained at high speed.